Role of the c/ebpβ mRNA intercistronic region in the STI571-dependent CEBPB expression in 32D-BCR/ABL cells. (A) Schematic representation of the expression vectors MigRI GFP WT-C/EBPβ (top) and MigRI GFP ΔuORF-C/EBPβ (bottom), in which the CEBPB-coding sequence is fused in-frame to the HA epitope at 3′. ATG1, 3, and 4 correspond to the initiation codon for LAP1, LAP2, and LIP, respectively. (B) Western blot analysis of HA-tagged CEBPB protein. Total lysate from 293T cells transfected with MigRI GFP WT-C/EBPβ-HA, used as positive control (lane 1), GFP-sorted 32D-BCR/ABL cells carrying the MigRI GFP empty vector (lane 2), GFP-sorted MigRI GFP WT-C/EBPβ cells (lanes 3-4), or MigRI GFP ΔuORF C/EBPβ cells (lanes 5-6) untreated (lanes 3 and 5) or treated (lanes 4 and 6) for 24 hours with 1 μM STI571 were probed with an antibody against the HA epitope. Equal loading was assessed by probing the filter with an anti-GRB2 antibody.